(CNN) -- Seth Rogen may be known for starring in some raunchy comedies, but there's one issue he doesn't take lightly: Alzheimer's disease.

The condition hits close to home because of his future mother-in-law, who is only 59 and has already had it for several years. And now Rogen, who has no family history of the disease, has become passionate about supporting his fiancee's family and spreading awareness of Alzheimer's.

"I think until you see it firsthand, it's kind of hard to conceive of how brutal it is," Rogen told CNN on Tuesday. "Until I saw it, you just don't get kind of how heartbreaking it can be."

Their story is just one example of the ripple effects of the emotional stress brought on from this disease.

"I just try to be as around as possible, as emotionally available as possible," Rogen told CNN Tuesday.

About 5.4 million people in the United States suffer from Alzheimer's disease, according to the Alzheimer's Association. By the middle of the century, 16 million people are expected to have the condition.

But the effect is even more far-reaching than just patients. There are nearly 15 million unpaid caregivers, not counting the rest of the family and friends who tirelessly provide support even though no prevention, treatment or cure has been found. The disease is fatal. The average length of the disease is four to eight years, although some people can live with it for 20 years.

Alzheimer's disease is the sixth leading cause of death in the U.S., and one in eight older Americans has it. Often the first symptoms include problems remembering names and recent events. As the disease progresses, the patient's confusion gets worse, and he or she might have difficulty with basic functions such as walking, speaking and swallowing. Some patients become violent, making caregiving even harder.

Given the family history of Rogen's fiancee, the cause seemed obvious when Miller's mother started showing symptoms. Both of Miller's grandparents had Alzheimer's disease; her grandfather showed signs in his 50s. Miller's mother was 55 when the family found out she had Alzheimer's disease.

"She'd forget a story that I'd told her, forget that she spoke with someone. So it was pretty clear to us what was going on," Miller told King.

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Most Alzheimer's patients are 65 or older, but the condition can strike much earlier. Having a parent, brother or sister with the condition raises your risk of developing it yourself; two first-degree relatives would raise that risk. There's also a particular gene that ups the chance that you'll get it. None of these factors is destiny, so it's hard to predict with certainty who will develop the disease.

"The biggest misconception is that it's this vague old-people disease that happens to old people," Rogen said.

Rogen, who has known Miller for six years, has also noticed her mother's decline. She had a lot of difficulty carrying on conversations and seemed to worsen even after a few months had gone by. She had more and more trouble accessing words, he told King.

Miller's father retired to take care of her at home. He cooks, cleans and does her hair every morning. Miller said her mother still knows her, but she struggles with Miller's father sometimes.

"He takes her shopping if she needs something to wear. And my dad does not like shopping," Miller said. "I don't think you are 62 years old and, you know, one day out of retirement think that you're going to be taking care of your spouse with Alzheimer's."

Miller's mother doesn't lose her temper, but she does get so confused that she doesn't know why people are trying to help her, the couple told King. Rogen told CNN that Miller's mother seems to get particularly disoriented after traveling from the East Coast to the West Coast, so Rogen and Miller try to go to her instead.

Having your relationship with a person change dramatically as he or she forgets events, information and even your name can be an emotional shock, says Beth Kallmyer, director of client and information services at the Alzheimer's Association.

Some people feel they don't know how to interact with an Alzheimer's patient, and withdraw because they don't want to see their friend or relative in such a state.

But Rogen doesn't feel uncomfortable with Miller's mother and described her as "easy to be around."

"I understand that she's going through something that I can't even conceive of what it's like to go through," Rogen said.

Besides providing emotional support for Miller, he has also given financial help to her family for medical and other expenses.

Caregivers should educate themselves about the disease and establish a support network, Kallmyer said.

"You can't do this alone. It takes too much out of you. You want to build a care team educated about the disease so that you know what to expect," she said.

Other people in Rogen's position, who want to help out but aren't the primary caregiver, should find out from the person closest to the patient what makes the patient happy and what makes him or her upset, Kallmyer said. They should also ask directly what the caregiver needs most.

Commonly, caregivers just need a break, she said. Often, a helpful gesture for a family dealing with this disease is to step in and do something with the Alzheimer's patient while the caregiver takes care of matters in his or her own life.

"One of the most important things a caregiver can do is ask for help," she said.

Talking about the disease has proved helpful for Miller's family, Rogen said.

"Once you get over the instinct to not talk about it, there are a lot of people who are a lot more supportive than you might imagine," Rogen said.